Method and apparatus for shock wave mitigation

ABSTRACT

A method and apparatus for mitigating blast compression waves is disclosed. The apparatus has a housing having an open end, and a piston slidably received in the open end of the housing in a substantially airtight engagement therewith. The piston and the housing define an interior wherein a compressible substance is confined. When a blast wave impacts the impact face of the piston and drives the piston toward the base of the housing, a shock wave is induced in the compressible substance. The shock wave is reflected by the base of the enclosure and the interior surface of the piston to mitigate the impact of the blast wave.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to commonly owned U.S. provisional application Ser. No. 60/721,798, filed Sep. 29, 2005, which is incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grants from the U.S. Army Research Office, Contract number W911NF-04-2-0011. The government does not have any rights in this invention.

TECHNICAL FIELD

The present invention relates generally to methods and apparatus for pressure wave mitigation and, more specifically, to methods and apparatus for blast wave mitigation.

BACKGROUND OF THE INVENTION

Conflicts throughout the World have already caused thousands of deaths of military personnel. A large percentage of these deaths are due to land mines and improvised explosive devices (IEDs). Explosion of land mines and IEDs send out blast waves of extremely high pressures, which destroy vehicles and incapacitate personnel. With the rise of low intensity conflicts involving terrorists and guerillas, Humvees, armored personnel carriers and tanks are forced into urban warfare for which they were not designed. In some instances, soldiers have taken it upon themselves to armor their vehicles with extra layers of scrap materials. While hardening may be effective against projectiles, it is not effective against blast wave impacts. The blast wave easily propagates through the hardened armor with little mitigation, killing occupants and damaging equipment.

Extensive research has been conducted to reduce the damage potential of blast waves. It is known that the damage potential of an explosive blast depends on three main factors: the force exerted on the target; the duration of the applied force; and the ability of the target to withstand the effects of the blast wave.

Two main approaches are used to mitigate the damage potential of the blast wave: blast absorbing materials and heterogeneous systems. For example, water has been used as a blast-absorbing material to reduce the damage caused by blast waves. One known water-based attenuation method uses a liquid layer confined within an elastic envelop to mitigate the blast wave. For this device, it was shown that the blast wave pressure attenuation coefficient depends on the distance from the blast epicenter to the point of measurement as well as the thickness of the water layer. The water medium delays the shock front and reduces the magnitude of initial peak shock pressure by approximately 40%-50%.

Other blast-absorbing materials used to mitigate the blast wave include both aqueous and metal foams. For aqueous foams, the vaporization of the liquid component has been shown to be detrimental to blast wave mitigation. Specifically, the many reflections off the foam/air interface produce a complicated waveform in the aqueous foam. Further, the blast mitigation behavior of cellular materials have been investigated. It is known that the transmitted pressure can be attenuated by the foam layer if the input blast load is below a critical value. Thus, this material can be used only for the lower pressure blast wave. For the high blast wave pressure, the cellular material will be destroyed and the pressure on the target will increase with adverse impact.

The list of blast absorbing materials typically includes granular, particulate matter, porous material, and foam. The momentum and energy of a blast wave can be absorbed by these “soft” condensed matters. The density, porosity and relative geometrical size of the so-called “soft” condensed matter are the main parameters determining the effectiveness of blast wave mitigation. For example, a tapered chain of elastic beads has been used for blast mitigation. The elastic beads act as an absorber of kinetic energy and can reduce it by about 30%. Results show that the energy absorption is affected by the restitution coefficient, the size of the particles and the tapering ratio. For particulate matter, the mitigation of an explosion is enabled largely by the consolidation of low density particulate matter into compacts of greater density. Mitigation effects decrease with average particulate size for particulate material with low areal densities.

In spite of their successful application to date, current methods and systems for using aqueous foams in pressure attenuating roles are inefficient and unnecessarily bulky.

Heterogeneous systems have also been designed to attenuate blast waves. For example, geometrical parameters of a blast wall have been studied to protect a target structure. A relationship has been demonstrated between blast mitigation and geometrical configuration of the wall. This relationship may be used to optimize the parameters of the blast wall. Results showed that the overpressures behind the wall are 30% to 60% of those without a wall. Solid barriers for shock wave containment or protection suffer from several shortcomings. Blast walls are typically massive and are thus inherently immobile and expensive. They cannot, therefore, be used in the majority of mobile applications.

In view of the shortcomings for existing apparatus and assemblies to mitigate blast shock waves as noted above, there has been found to remain a need for an assembly for more effectively mitigating blast waves.

SUMMARY OF THE INVENTION

The present invention generally relates to an assembly for mitigating a blast compression wave. The assembly comprises a housing having a base wall, an outwardly extending wall, and an open end. A piston having an impact face and an interior face is slidably received in the housing in a substantially airtight engagement therewith. The piston, the base wall and the outwardly extending wall define an interior. A compressible substance is confined within the interior, whereby when the blast wave impacts the impact face of the piston and drives the piston toward the base of the housing, a shock wave is induced in the compressible substance. The shock wave is reflected by the base of the enclosure and the interior surface of the piston to mitigate the impact of the blast wave.

In accordance with another embodiment of the present invention, a method for mitigating blast compression waves is disclosed. The method includes the step of providing a housing having a base wall and an outwardly extending wall, and a piston slidably received in the housing and in substantially sealing engagement therewith. The housing and piston defining an interior. The method further includes the step of inducing a shock wave in the interior of the housing through the piston receding into the interior upon the impact of the blast wave. The shock wave is reflected within the interior of the housing.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in detail below with reference to the attached drawing figures, wherein:

FIG. 1 is a schematic of the blast wave mitigation apparatus of the present invention;

FIG. 2 is a schematic of an exemplary embodiment of the blast wave mitigation apparatus for a testing of its effectiveness;

FIG. 3 is a table listing the parameters of the blast wave mitigation apparatus for the embodiment of FIG. 2;

FIG. 4 is a diagram illustrating the relationship between pressure and the time of impact on the piston of the blast wave mitigation apparatus of the present invention;

FIG. 5 is a diagram illustrating the difference of indentation between a control and the exemplary embodiment of the blast mitigation apparatus of the present invention; and

FIG. 6 is a diagram illustrating the relationship between pressure and the time of impact on the base wall of the exemplary embodiment of the blast wave mitigation apparatus of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an apparatus for blast wave mitigation, generally indicated at 10. The apparatus 10 comprises a piston 12 slidably received in and in substantially sealing engagement with a housing 14, such as in a piston-cylinder assembly. The housing 14 may be a cylindrical housing having a base 16 and a cylindrical wall 18 extending outwardly from the base wall. Alternatively, the housing may be generally rectangular in shape having opposing side walls and opposing upper and lower walls extending outwardly from the base wall 16. The housing 14 is preferably made of steel, such as rolled homogeneous armor steel. It is to be understood by those skilled in the art that other materials may be selected without departing from the scope of the present invention.

The outwardly extending cylindrical wall 18, or alternatively opposing side walls and upper and lower walls, may extend outwardly for different lengths from the base depending on the particular requirements of the blast wave mitigation device 10. In an exemplary embodiment, the length the cylindrical wall extends from the base is between approximately 1 to 20 cm. More specifically, the length is more preferably between approximately 1.5 to 10 cm. It is to be understood that the particular length dimension may differ and still be within the scope of the present invention.

The piston 12 is operably configured to be slidably received within the cylindrical housing in a substantially airtight engagement. The piston 12, wall 18, and base 16 together define an interior 20 of the piston-cylinder assembly 10. The piston includes a blast-impact face 22 and an interior face 24. Depending on the materials selected and the particular usage of the blast mitigation apparatus 10, the piston may have a thickness of between about 1 to 10 cm. and preferably a thickness of about 5 cm. Preferably, the piston is likewise made of rolled homogeneous armor steel. It is to be understood by those skilled in the art that other materials may be selected and that the piston may present numerous face configurations and thicknesses without departing from the scope of the present invention.

The interior 20 is filled with air or other inert gases having desirable characteristics. Blast waves, when traveling through air or other gases, produce increases in pressure (referred to as “overpressure”), temperature; and also accelerate gas molecules in the direction of wave travel. For all blast waves, the wave speed, overpressure, and temperature increase they induce in the local medium are mathematically linked.

Having described an exemplary embodiment of the present invention, an exemplary operating environment for the present invention is described. A continuous grid of the blast wave mitigation apparatus 10 of the present invention may be used to cover the surface of military structures. The base 16 of the housing 14 is mounted to the structure through known means. As a blast wave impacts the blast impact face 22 of the piston 12, the piston is forced to recede into the interior 20 of the assembly 10. This piston motion induces a weaker shock wave that propagates toward the base 16 of the device 10 at supersonic speed. When the shock wave impacts the base 16, it is reflected back and travels toward the interior face 24 of the piston 12. When the reflected shock wave hits the interior face 24 of the piston, it is reflected again. This process repeats until the piston 12 comes to a complete stop. Each time the shock wave is reflected, the pressure of the gas in the interior of the assembly 10 increases. The pressure reaches its maximum when the piston 12 comes to rest. The repeated reflection of the shock wave within the blast mitigation device 10 significantly increases the duration of the force on the base 16 of the housing 14 as compared to the duration of the blast wave alone. Because the impulse of the blast wave is almost conserved, this results in a significant decrease of the force on the base of the cylinder. The duration of the force on the base of the device is increased to several orders of magnitude of the duration of the blast wave, and, thus decreases the maximum pressure on the base and the surface to which it is mounted by several orders of magnitude.

The pressure on the base of the blast wave mitigation device is the key parameter determining the effectiveness of the device.

EXAMPLE

In an effort to determine the effectiveness of the blast mitigation device 10 of the present invention, the blast mitigation device was setup as shown in FIG. 2. A honeycomb structure, which can withstand a peak pressure of about 2 MPa, is placed on a steel test platform. The honeycomb structure was covered by either the blast wave mitigation device 10 of the present invention or a control device. The control device has the same dimensions and weight as the blast wave mitigation device. A rolled homogeneous armor steel plate is used to protect the honeycomb structure that is not covered by the blast wave mitigation and control device 10. A hole was cut at the center of the steel plate to expose the blast wave mitigation device and control device. The diameter of the hole is the same as that of the blast wave mitigation device. The steel plate has a thickness of approximately 9 cm. The blast wave is generated by detonating 1.36 kg of Pentolite at a distance of 0.26 m. This setup is capable of testing the effectiveness of the blast wave mitigation device qualitatively.

The design parameters of the blast wave mitigation device 10 used with the honeycomb structure are listed in the Table of FIG. 3. The parameters that significantly affect the effectiveness of the blast wave mitigation device are the thickness of the piston 6 and the length of the cylinder L. D is the diameter of the piston. The control device has the same dimensions and weight as the blast wave mitigation device. The blast wave mitigation device has a square base 16 of approximately 460×460 mm. One experiment was conducted with the control device and one with the blast wave mitigation device.

Based on compiled experimental data, the peak blast wave pressure generated by 1.36 kg Pentolite is approximately 140 MPa and the duration of the blast wave is roughly 0.2 ms. Experimental data of the blast wave pressure as a function of time is shown in FIG. 4. When the Pentolite is detonated, the blast wave was transmitted through the control device with negligible attenuation. Since the blast wave pressure significantly exceeds the pressure rating of the honeycomb structure, the impact of the blast leaves an indentation of roughly 13 mm in the honeycomb structure, when the control device is used (see FIG. 5).

When the blast wave mitigation device 10 in accordance with the present invention is used, the impact of the blast wave caused the piston 12 to recede. The piston movement induced a weak shock wave inside the blast wave mitigation device 10. The shock wave propagated inside the blast wave mitigation device and was reflected repeatedly. Each time, the shock wave was reflected, the pressure, temperature and density increased. The increased pressure slowed down the piston, which eventually came to a complete stop. At this moment, the pressure on the base of the device reached its maximum. The shock wave propagation process inside the device lengthened the duration of the force on the base of the device to several orders of magnitude of the duration of the blast wave, while it decreased the maximum pressure by several orders of magnitude.

FIG. 7 shows that the peak pressure on the base of the blast wave mitigation device was predicted to be slightly higher than 2 MPa. The reduction of the peak blast wave pressure was predicted to be over 97%. As a result, the honeycomb structure should be adequately protected by the blast wave mitigation device. This was confirmed experimentally; the blast wave impact left a very shallow indentation along the edge of the blast wave mitigation device, which is where the stress concentration occurred. The indentations on the honeycomb structures with the blast wave mitigation device and the control device under the impact of the blast wave are shown in FIG. 5. The experimental results prove qualitatively that the blast wave mitigation device is effective in mitigating the blast wave impact.

It is to be understood that the specific embodiments of the present invention that are described herein is merely illustrative of certain applications of the principles of the present invention. It will be appreciated that, although an exemplary embodiment of the present invention has been described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Therefore, the invention is not to be limited except as by the appended claims. 

1. An assembly for mitigating a blast compression wave, the assembly comprising: a housing having a base wall, an outwardly extending wall, and an open end; a piston comprising an impact face and an interior face, the piston being slidably received in the housing in a substantially airtight engagement therewith, the piston, the base wall and the outwardly extending wall defining an interior; and a compressible substance confined within the interior, whereby the blast wave impacts the impact face of the piston and drives the piston toward the base of the housing thereby inducing a shock wave in the compressible substance within the interior, the shock wave being reflected by the base of the enclosure and the interior surface of the piston to mitigate the impact of the blast wave.
 2. The apparatus of claim 1 wherein the piston is formed of steel.
 3. The apparatus of claim 1 wherein the piston has a thickness from the blast-impact face to the interior face of between approximately 1 centimeters to approximately 10 centimeters.
 4. The apparatus of claim 1 wherein the area of the blast impact face of the piston is between approximately 0.1 meters and 0.15 meters.
 5. The apparatus of claim 1 wherein the mass of the piston is between approximately 40 kilograms and approximately 60 kilograms.
 6. The apparatus of claim 1 wherein outwardly extending wall is cylindrical and extends outwardly from the base wall between approximately 0.05 meters and 0.2 meters.
 7. The apparatus of claim 1 wherein outwardly extending wall comprises opposing side walls and opposing upper and lower walls.
 8. The apparatus of claim 1 wherein the impact face of the piston is substantially planar.
 9. The apparatus of claim 1 wherein the base wall is substantially planar.
 10. A method for mitigating blast compression waves, the method comprising the steps of: providing a housing having a base wall and an outwardly extending wall, and a piston slidably received in the housing and in substantially sealing engagement therewith, the housing and piston defining an interior; inducing a shock wave in the interior of the housing through the piston receding into the interior upon the impact of the blast wave; and reflecting the shock wave within the interior of the housing.
 11. The method of claim 10 further comprising the step of increasing the duration of the force upon the base wall of the housing as compared to the duration of the force from the blast wave.
 12. The method of claim 10 wherein the step of reflecting the shock wave within the interior of the housing comprises the step of repeatedly reflecting the shock wave between the base wall and an interior face of the piston until the piston is stopped and the shock wave attenuated.
 13. The method of claim 10 wherein the step of providing a housing and a piston slidably received in the housing and in substantially sealing engagement therewith, the housing and piston defining an interior further comprises the step of providing a compressible gas within the interior.
 14. A method for mitigating blast compression waves, the method comprising the steps of: providing a housing having a base wall and an outwardly extending wall, and a piston slidably received in the housing and in substantially sealing engagement therewith, the housing and piston defining an interior; conserving substantially the impulse of the blast wave within the interior of the housing and piston; and increasing the duration of the force upon a base wall of the housing. 